# std::identity

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< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
struct identity; |  | (since C++20)

`std::identity` is a function object type whose operator() returns its argument unchanged.

### Member types

Type | Definition
`is_transparent` | unspecified

### Member functions

operator() | returns the argument unchanged (public member function)

## std::identity::operator()

template< class T > constexpr T&& operator()( T&& t ) const noexcept; |  |

Returns std::forward<T>(t).

### Parameters

t | - | argument to return

### Return value

std::forward<T>(t).

### Notes

`std::identity` serves as the default projection in constrained algorithms. Its direct usage is usually not needed.

### Example

Run this code

    #include <algorithm>
    #include <functional>
    #include <iostream>
    #include <ranges>
    #include <string>

    struct Pair
    {
        int n;
        std::string s;
        friend std::ostream& operator<<(std::ostream& os, const Pair& p)
        {
            return os << '{' << p.n << ", " << p.s << '}';
        }
    };

    // A range-printer that can print projected (modified) elements of a range.
    template<std::ranges::input_range R,
             typename Projection = std::identity> //<- Notice the default projection
    void print(std::string_view const rem, R&& range, Projection projection = {})
    {
        std::cout << rem << '{';
        std::ranges::for_each(
            range,
            [O = 0](const auto& o) mutable { std::cout << (O++ ? ", " : "") << o; },
            projection
        );
        std::cout << "}\n";
    }

    int main()
    {
        const auto v = {Pair{1, "one"}, {2, "two"}, {3, "three"}};

        print("Print using std::identity as a projection: ", v);
        print("Project the Pair::n: ", v, &Pair::n);
        print("Project the Pair::s: ", v, &Pair::s);
        print("Print using custom closure as a projection: ", v,
            [](Pair const& p) { return std::to_string(p.n) + ':' + p.s; });
    }

Output:

    Print using std::identity as a projection: {{1, one}, {2, two}, {3, three}}
    Project the Pair::n: {1, 2, 3}
    Project the Pair::s: {one, two, three}
    Print using custom closure as a projection: {1:one, 2:two, 3:three}

### See also

type_identity (C++20) | returns the type argument unchanged (class template)

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